1. Field of the Invention
The present invention relates to a circuit arrangement for a line test and for feeding a corrosion protection current into a two-wire line, in particular for use in switching centres for digital telecommunications networks.
2. Description of the Prior Art
Nowadays, telephone networks which were originally designed for telephone applications are increasingly being used exclusively for digital data traffic. The existing two-wire lines are in this case used for transmission of radio-frequency signals which, for example, are at up to 30 MHz for applications. At the switching centre end, DSL systems are coupled to the lines and, in addition to the task of communication organization, also have to test the quality of the two-wire lines.
Possible faults on two-wire lines which can interfere with signal transport are, inter alia, bridged line pairs or individual lines, or single lines of a two-wire line, connected to earth. Line faults such as these change the predetermined impedance of the two-wire line and can thus be identified by means of impedance measurements by the DSL system. In this case, balanced faults such as the bridging of the two individual lines are difficult to distinguish from unbalanced faults, such as an undesirable connection between one of the individual lines via a resistance to earth, since the DSL system is generally coupled to the two-wire line only non-conductively, via a transformer.
In addition to the line qualification and line test of the two-wire lines by means of the switching centre, a corrosion protection current should also be fed into the conductor loops formed from the two-wire lines, at regular time intervals. A corrosion protection current such as this, which is also known as a sealing current, is used to break through thin oxide layers or other layers that constrict the current flow at junction points between line sections on lines which are composed, for example, of copper, in order to ensure a reliable electrical connection over the entire respective conductor loop. For present-day telecommunications networks, standard requirements of 1.5-20 mA are quoted for a direct current such as this for corrosion protection.